1. Field of the Invention
The present invention relates to a power unit for driving driven parts for propelling a transport machine.
2. Description of the Related Art
Conventionally, as a power unit of this kind, one disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2000-197324 is known. This power unit is for driving drive wheels of a vehicle, and is provided with an internal combustion engine, and first and second rotating machines. The first rotating machine has a stator formed by a plurality of armatures, an intermediate rotor formed by winding coil around an iron core, and an inner rotor formed by a permanent magnet. The stator, the intermediate rotor, and the inner rotor are arranged in this order from the outside to the inside in a radial direction. Further, in the first rotating machine, an induction machine is formed by the stator and the intermediate rotor, and a synchronous machine is formed by the intermediate rotor and the inner rotor. The above-mentioned second rotating machine is a general one-rotor type, and has a stator formed by a plurality of armatures, and a rotor formed by a permanent magnet.
The intermediate rotor of the first rotating machine is mechanically connected to the crankshaft of the engine, and the inner rotor of the same to the rotor of the second rotating machine, and a rotor of the second rotating machine is mechanically connected to the drive wheels. Further, the stators of the first and second rotating machines are electrically connected to a battery via a first controller and a second controller, each formed by e.g. an inverter.
In the conventional power unit constructed as described above, during running of a vehicle on which the power unit is installed, the power of the engine is transmitted to the drive wheels, while changing the speed thereof in the following manner: When the rotational speed of the engine is higher than that of the drive wheels, part of the power from the engine is used to generate electric power by the first rotating machine. This converts the part of the power from the engine into DC power by the stator of the first rotating machine, whereby power generation is performed, and at the same time, the remainder of the power from the engine is electromagnetically transmitted to the inner rotor via the intermediate rotor, and then is transmitted to the drive wheels. The electric power thus generated is supplied to the stator of the second rotating machine via the first and second controllers, and the power thus generated by the rotor of the second rotating machine is transmitted to the drive wheels. During this process, by controlling the electric power generated by the first rotating machine and the power of the second rotating machine, the power of the engine is transmitted to the drive wheels while steplessly changing the speed thereof.
As described above, in the conventional power unit, transmission of power of the engine to the drive wheels is performed via a first path formed by the intermediate rotor, magnetism, and the inner rotor, and a second path formed by the intermediate rotor, magnetism, the stator of the first rotating machine, the first and second controllers, the stator of the second rotating machine, magnetism, and the rotor of the second rotating machine. In the first path, the power of the engine is transmitted by a so-called magnetic path formed by magnetism generated in the intermediate rotor, and hence it is possible to obtain a relatively high transmission efficiency. On the other hand, in the second path, the power of the engine is transmitted via a so-called electrical path in which the power is once converted into DC power, and is then converted back into power (kinetic power), so that conversion loss of the inverter or loss by generation of Joule heat occurs to make the transmission efficiency via this path lower than that via the magnetic path.
In the conventional power unit, due to the arrangement thereof, approximately half of the power of the engine is transmitted to the drive wheels by the electrical path via the second path, and hence the driving efficiency of the power unit becomes lower. For the same reason, the second rotating machine is increased in size to increase its weight and manufacturing costs. Further, since the induction machine is formed by the stator and the intermediate rotor, when the electric power is generated by the first stator, Joule heat is generated not only in the coil of the first stator but also in the coil of the intermediate rotor, which prevents the sufficient power generation efficiency from being obtained, to further lower the driving efficiency of the power unit. Further, since the intermediate rotor is formed by winding coil around the iron core, the durability of the intermediate rotor is relatively low, and in turn, the durability of the whole power unit is also low. Further, there is a fear that the intermediate rotor, which is so low in durability, cannot withstand the repeated transmission of the power from the engine connected thereto, and becomes incapable of properly transmitting the power to the drive wheels.